This invention relates to method and apparatus for coating detection and surface evaluation. It pertains particularly to method and apparatus for detecting and evaluating coatings on pieces of material, which coatings because of their color or invisibility are difficult or impossible to detect and evaluate by visual observation.
In the photographic arts it is frequently necessary to identify the emulsion side of a photographic film or negative. For example, when making prints from photographic slides it is necessary to know which side of the slide is the emulsion side. If a mistake is made, and the slide is not oriented properly, during the duplicating operation, the picture will be reproduced backwards, i.e. the left side of the picture becomes the right side, and vice versa. If this occurs, the slide must be reprinted. This multiplies the operating cost because of requirements of individual handling, lost time in matching the reprint with the customer's order, scheduling disappointments and the like.
In the optical arts it also is necessary at times to determine quickly on which side of a coated base the coating occurs. This may be difficult, because to the human eye both sides appear the same. This is the case, for example, in the case of an optical grade beam splitter, or a mirror, wherein the glass has been surface coated.
Another instance in which it is desirable to verify the existence of a coating occurs in the metal processing arts, wherein metal surfaces are coated with various materials. For example, the surface of an opaque metal such as steel, aluminum, brass, etc. may be coated with a clear, transparent lacquer to produce a coating which is invisible to the unaided eye. The same situation exists when the metal base is coated with a material that is the same color as the metallic surface. This might occur when a silver or aluminum coating is applied to a silver or aluminum surface.
It is possible, of course, to solve the foregoing problem by scratching the surface to see if some of the coating material shaves off, or by applying a chemical to cause the coating to alter in some visible manner. Such procedures degrade or destroy the subject of the test, and are not usually commercially acceptable.
The need accordingly exists to provide method and apparatus for determining qualitatively on which side of a coated object the coating occurs. The need further exists to determine the condition of such a coating, i.e. whether it is a thick or a thin coating, or whether the coating is continuous or discontinuous.
It is the general purpose of the present invention to provide method and apparatus for achieving these ends.
Further objects of this invention are the provision of method and apparatus for determining the coating condition of a coated object which method and apparatus are qualitatively accurate; applicable to a wide variety of coated objects; applicable broadly to coated objects which may be either transparent, translucent or opaque; applicable to the detection of very thin coatings; simple, quick and easy to use; adaptable to automation; free from any tendency to damage either the coating or the substrate; and adaptable for use by unskilled operators.
I have discovered that the coated and uncoated surfaces of a wide variety of substrates have differing properties of transmission or reflectance of light. Accordingly, the foregoing and other objects of my invention may be accomplished by the provision of a method which broadly comprises directing a primary beam of light against the surface of the test piece, under conditions predetermined to generate a secondary beam of light. The secondary beam is either transmitted or reflected, depending upon whether the piece is transparent, translucent or opaque to the primary beam.
The secondary beam is directed against light intensity measuring means which measures the intensity of the secondary beam. The procedure is repeated on a different area of the test piece and the results compared. This determines not only the presence or absence of a coating, but also its quality i.e. whether it is a thick coating, a thin coating, a continuous coating or a discontinuous coating.
I have discovered further that the light transmissive and reflecting qualities of coated and uncoated surfaces may be intensified and made more readily observable by the expedient of applying to such surfaces a puff of condensible vapor. Many coated surfaces tend to absorb or diffuse the vapor. Other coated surfaces, under the same conditions of vapor application, tend to condense the vapor. The uncoated base surfaces have different capacities for vapor condensation to do the coated surfaces.
The resulting fogged surfaces have different qualities of light transmission and reflectance than the unfogged surfaces, to a very marked degree. This affords a positive and accurate tool for determining coating presence. It also provides a means of determining surface properties, since thick coatings and continuous coatings have greater capacity for dissipating the vapor, and vice versa.